Preparation,Properties And Photocatalytic Application Of Polyoxometalates-based Semiconductor Composite Materials

Photocatalytic technology has a high status in wastewater prevention and control due to its green efficiency,mild conditions and convenient operation.The core of successfully degrading pollutants is to find semiconductor materials with excellent performance by using photocatalytic technology.Generally,materials that are green,environmental friendly,inexpensive,non-toxic,and free of secondary pollution are selected.The lack of light absorption of single-phase semiconductor photocatalysts and the rapid recombination of charge carriers greatly limit the photocatalytic activity.Therefore,it is an important topic to explore how to perform efficient photocatalysis.The recombination between semiconductors is an effective way.The semiconductor-like properties of polyoxometallates（POMs）give them the ability to modify semiconductor materials,which can effectively transfer photo-generated electrons and holes in semiconductor materials,promote the efficiency of carrier separation,and thereby inhibit photo-generated electron-hole recombination.Polyacid-based derivative materials also have good photocatalytic activity.This paper discusses three works,the contents are as follows:1.Using Ce₂S₃ as the core photocatalyst and Mo/W mixed Keggins heteropolyacid H₃PMo_xW_12-xO₄₀(PMo_xW_12-x)as the modified component,we prepared binary composite photocatalyst Ce₂S₃@PMo_XW_12-X（X=3、6、9）containing different polyacids and different doping amounts.After studying their photoelectric properties,the best binary composite photocatalyst with the best photoelectric properties was selected as Ce₂S₃@0.5%PMo₃W₉.On this basis,in order to improve the conductivity of inorganic materials,the conductive polymer polypyrrole was added to prepare PPy/Ce₂S₃@0.5%PMo₃W₉ ternary composite photocatalyst.The photochemical properties of the ternary composite photocatalyst was explored and used for the photocatalytic reduction of Cr（VI）and photocatalytic oxidation of Rh B.In the prepared PPy/Ce₂S₃@0.5%PMo₃W₉ nanometer materials,POMS is a kind of nano particles,which is helpful for charge separation and electron transfer.The composite of Ce₂S₃ and PMo₃W₉ improves the utilization rate of visible light.The addition of PPy not only improves the electrical conductivity of the composites,but also prevents the coating of Ce₂S₃ from the oxidation of S^2-into the S element,thus preventing the occurrence of photocorrosion.The reasonable matching of the three energy band structures makes the photocatalytic performance of PPy/Ce₂S₃@0.5%PMo₃W₉significantly better than Ce₂S₃@0.5%PMo₃W₉ and Ce₂S₃.2.Considering that WO₃/TiO₂ is a heterojunction photocatalyst with excellent performance,the heterojunction prepared by pure WO₃ and TiO₂ precursor system will lead to the problems of large grain boundary area,not close contact and more defects.Therefore,we explored the method of calcining polyacids to prepare WO₃/TiO₂ heterojunction to increase the close contact between WO₃ and TiO₂ grain boundaries.First,Ti₆-PW₁₂ was synthesized by co-precipitation,and then WO₃/TiO₂ heterogeneous materials were prepared by calcination process.The XRD spectra revealed that the main constituents after calcination were WO₃ and TiO₂.The UV-Vis spectrum of solid state shows that the light absorption capacity of the calcined material is enhanced and the band gap is reduced.I-t,EIS,SPS and PL data determined that WO₃/TiO₂heterojunction photocatalyst was obtained after calcining the polyacid precursor,and the WO₃/TiO₂ heterojunction was obtained by sintering the precursor at 700℃showed the best photocatalytic performance.This may be because the WO₃/TiO₂ heterogeneous crystal surfaces are in closer contact with each other after calcination of polyacids,the crystal surface defects are reduced,the recombination of electron-hole was effectively inhibited.The photocatalytic degradation of tetracycline hydrochloride by WO₃/TiO₂ heterojunction material prepared by calcination was investigated under visible light irradiation.The optimal degradation rate was86%by optimizing the experimental parameters.At the same time,the mechanism of photocatalytic activity showed that h⁺and·O₂^-were the main oxidizing substances,and they could quickly decompose tetracycline.3.Considering the potential semiconductor properties and excellent photocatalytic performance of heteropolytungstate containing titanium,a novel water-soluble titanium-containing polyacid was synthesized by using sodium tungstate and titanium-oxide sulfate as precursors in conventional aqueous solution.The molecular formula is:Na₂H₁₀[Co₄（H₂O）₂(TiW₉O₃₄)₂]·48H₂O（1）.The crystallographic data of compound 1 shows that 1 has a typical sandwich framework,which is composed of two Keggin-type[TiW₉O₃₄]monomers connected through an intermediate layer[Co₄O₁₄（H₂O）₂].Sodium ions not only act as counter cations,but also connect with polyacid anions to form a 2D layered structure.These subunit units are extended by sodium ions to form a grid structure.At the same time,the data reveals the semiconductor properties of 1,in addition,we explored its functional applications for photoelectric sensing of dopamine.